Recent medical ultrasound advances have produced ultrasound imaging systems that incorporate coherent receive beamformers. As used herein, two signals are coherent when sufficient information is stored, preserved, or maintained to enable accurate characterization of the relative amplitude and phase of the complex envelopes of the two signals. Signals from individual transducer elements, as well as signals that are summed from signals from individual transducer elements, can be coherent. Coherent receive beamformers permit post-acquisition generation of multiple lines of ultrasound data and are described in U.S. Pat. Nos. 5,476,098 and 5,623,928, the latter of which is assigned to the assignee of the present invention. These systems are still limited, however, by the need to transmit acoustic energy into the imaged object in the form of externally generated ultrasound beams (e.g., focused beams, unfocused beams, or multiple spherical or cylindrical beams from individual point or line acoustic sources, respectively). The time required for the transmitted acoustic energy to propagate through an object limits data acquisition rates and frame rates of displayed images. Data acquisition rates and frame rates are further limited in two-dimensional imaging (image planes, static or versus time), three-dimensional imaging (static volume images), and four-dimensional imaging (three-dimensional volume images versus time), where multiple transmit/receive sequences are required to collect data. Additionally, coherent beamforming techniques are sensitive to motion artifacts that are introduced during multiple transmit/receive sequences.
Another recent medical advance exploits the Hall effect to examine local conductivities of tissues and generate ultrasonic pulses within the tissue, as described in Ehrenstein, "New Technique Maps the Body Electric," Science, Vol. 276, page 681, May 2, 1997; Wen et al., "Hall Effect Imaging," Laboratory of Cardiac Energetics, National Heart, Lung and Blood Institute; and "Researchers create Hall effect imaging," Biophotonics International, pages 34-35, July/August 1997. As described in these references, when an electric field pulse is applied to an object in the presence of a strong magnetic field, regions within the object respond to the electric field with a current density proportional to the local apparent conductivity. At the interfaces between regions of differing conductivities, the current densities (and the Lorentz forces on the currents) become discontinuous. The discontinuities of the Lorentz forces result in ultrasound pulses emanating from the interfaces. The references suggest that emitted pulses and conductivity profiles can be detected with fiber-optic ultrasonic sensors and photoacoustic transducers, which avoid interference with the magnetic field. As described in Wen, these transducers receive emitted pulses when the pulses fall within the beam profile of the transducer, requiring multiple transmit events to form an ultrasound image.
There is a need, therefore, for an ultrasound imaging system and method that take full advantage of these recent medical advances.